The secret is in the leaves, and more particularly, the levels of formyl phloroglucinol compounds (FPCs), that naturally occur in eucalyptus leaves. These FPC's protect the plant from predators, but they are also toxic, and deter grazers like koalas. The prediction is that the more FPC's a tree has, the less likely a koala will stop by for lunch. Levels of FPC's vary not only among different species of eucalypts, but also between different individuals of the same species, so determining if a forest is suitable can be very tricky.

Currently koalas are being relocated in Victoria from French Island and Sandy Point to a number of new habitats, including Fraser National Park and the Ballarat area. However, biologists are fast running out of known suitable places to move the koalas.

PhD student Ben Moore and post-doctoral fellow Dr Ian Wallis, from the Division of Botany and Zoology in the ANU Science Faculty, have developed a quick way of measuring FPC concentration across a whole forest canopy. They've correlated this with studies about how much FPC koalas are willing to stomach, to come up with an index for identifying potential koala habitats.

Mr Moore used near infrared (NIR) spectroscopy to determine concentration of FPC in leaves, which in turn showed how palatable they are.

"I can now predict intake of other trees just by scanning the leaves, which is a very quick and inexpensive method," he said.

The next step will be to test the method, by correlating what the NIR has picked up, with what the koalas are actually doing in the trees.

Koalas on Phillip Island, off the coast of Victoria, will be fitted with radio collars which have a microphone to pick up their chewing activity. They will be released into an area of forest which has already been scanned for FPC concentration.

These observations will measure exactly what leaves the koalas are feeding on, allowing confirmation of the NIR technique.

"A lot of people are studying the ecology of koalas and doing radio tracking or just observing a koala in a tree," Dr Wallis said. "There are a lot of reasons, however, why a koala might be using a tree, not just for feeding."

By using a collar fitted with a microphone, the koala behaviour will be closely monitored. "We will be able to hear a lot of nothing, a lot of koala snoring, but also we will be able to hear koalas chewing," Mr Moore said.